1. Field of the Invention
The present invention relates to power supplies in general and, more particularly, to power supplies that are synchronized to an external frequency.
2. Prior Art
The use of power supplies as an energy source for different types of loads is well known in the prior art. Usually, power supplies may be classified based upon the topology and/or the frequency of operation. Among the various types of power supplies the flyback or blocking oscillator type is widely used for lower power applications. This type of power supply is popular because it has a relatively simple topology, it requires a reduced number of components, it provides multiple regulated outputs from a single supply, it has high converter efficiency, etc.
A typical flyback power supply is a switching regulator that stores energy in a magnetic field, usually in a power transformer, and then transfers this stored energy into a load. Usually, a first DC potential or voltage is converted by the power transformer into one or more other DC output voltages. By varying the amount of energy stored and dumped per cycle, the output power can be controlled and regulated. Energy variation is achieved by a high power switching transistor, connector in series with the primary winding of the power transformer. The "On-Time" and "Off-Time" of this transistor control the amount of energy coupled across the power transformer. Thus, if the power transistor is off, current flows in one or more of the secondary windings and energy is delivered to the load prior to initiating another cycle. If the power transistor is "on", current flows through the primary winding of the power transformer and energy is stored in the transformer.
Even though the blocking oscillator power supply topology is attractive, it operates at variable frequencies. There are several applications in which it is desirable to synchronize the operating frequency of the blocking oscillator power supply to an external frequency. Such synchronizing is particularly important if interference is likely to occur between the operating frequency of the power supply and the device (load) which it drives. Computer type loads and CRT (Cathode Ray Tube) displays are usually susceptible to such interferences. With CRT displays, the interference usually affects the picture quality.
One example of prior art attempts to adjust the frequency of a switched power supply is set forth in IBM Technical Disclosure Bulletin, Vol. 29, No. 11, April 1987 (page 4948). In the article a pulse width modulator (PWM) controlled switch mode power supply is synchronized to run at half the horizontal scan rate of a CRT display. Even though this approach is a step in the right direction, the power supply is operated as a non-reset flyback switcher. This type of switcher is plagued with operational problems including high energy dissipation noise, etc.
Other types of circuits and techniques for adjusting the operational frequency of a switching power supply are set forth in U.S. Pat. Nos. 4,326,244; 4,323,961; 4,510,563 and 4,525,674.